The correlation of morphologically distinct immunocytes with a specific set of functions is necessary in order to evaluate age associated immunodeficiency. Use of an anti-B cell antisera has shown that the spleens from old mice, while performing well in a "B" mitogenic assay, actually do not have normal B cells. Several B cell criteria change with age and quantitation of B cells must be done carefully. Studies in the whole animal have shown a decreased tumor immunity along with a poor T cell function which may be able to be correlated.